How to Set the /3GB Startup Switch in Windows

Topic Last Modified: 2006-04-19

Windows Server 2003 includes support for a startup switch that lets you tune the allocation of use of memory and memory address space. Regardless of the amount of physical memory in your system, Windows uses a virtual address space of 4 GB, with 2 GB allocated to user-mode processes (for example, applications) and 2 GB allocated to kernel-mode processes (for example, the operating system and kernel-mode drivers). On systems that have 1 GB or more of physical memory, these two startup switches can be used to allocate more memory to applications (3 GB) and less memory to the operating system (1 GB). This additional virtual address space helps reduce the amount of memory fragmentation in the virtual address space of the Exchange information store process.

The /3GB switch is used to effect this allocation change. The switch is entered in the system’s boot.ini file and takes effect after a restart.

In Windows Explorer, navigate to the system partition. This is the partition that holds hardware specific Windows files such as Boot.ini and NTLDR.

If you cannot see the Boot.ini file, it could be because the folder options are set to hide protected operating system files. If this is the case, in the Explorer window, click Tools, Folder Options, and then click View. Clear the Hide protected operating system files (Recommended) check box. When prompted, click Yes.

After the Boot.ini file is visible in Explorer, right-click the file, click Open With, and then click Notepad to open the file.

Add the /3GB switch to the end of the line starting with "multi" under the [Operating Systems] section.